1. Field of the Invention
The present invention concerns a method for time synchronization of different components of a magnetic resonance system which interact in an acquisition sequence, and a magnetic resonance system for implementing such a method.
2. Description of the Prior Art
Magnetic resonance tomography is an imaging modality that is used for examination and diagnosis in many fields of medicine. It is based on the physical phenomenon of nuclear magnetic resonance. To acquire magnetic resonance (MR) signals, in an MR system, a static basic magnetic field is generated in the examination region, in which the nuclear spins or the magnetic moments of the atoms in the examination subject align. By means of an MR acquisition sequence, the nuclear spins can be deflected or excited out of the aligned position (i.e. the rest position) or another state by the radiation of radio-frequency pulses. The excited spin system can have a temporal dynamic.
The phase evolution of the spin system in the slice is described by the coherence curve. If the spins of a spin system of a defined slice all have an identical phase position, a magnetization signal with a large amplitude can be detected. A relatively strong signal can be detected since no destructive interference exists between the signals of different spins of different phase.
By applying a slice selection gradient upon radiation of the radio-frequency pulses, only nuclear spins are excited in a slice of the examination subject in which the resonance condition is satisfied due to the local magnetic field strength. Such a spatial coding can take place by application of a phase coding gradient as well as a frequency coding gradient during the readout. By slice-selective excitation it is possible to acquire MR exposures of multiple slices of an examined person.
Modern MR systems typically operate with distributed controls for the individual portions of an MR acquisition sequence. For example, a radio-frequency (RF) transmission system can include an RF generator and an amplitude modulation unit that respectively generate the radio-frequency portion or, respectively, the radio-frequency and the low-frequency amplitude modulation or, respectively, envelope of the RF pulses. A gradient system can generate the gradient fields for spatial coding.
The time synchronization of these components relative to one another is a basic requirement for a high quality of the MR imaging. With regard to conventional MR imaging it is already necessary to achieve such a high degree of time synchronization. However, there are additional fields of application in which the time synchronization is particularly important. For example, there is a significant interest in implementing the scanning scheme of k-space not only according to Cartesian MR imaging sequences but rather also according to a non-Cartesian scanning. Non-Cartesian scanning schemes have advantages, for example with regard to higher signal-to-noise ratio or better robustness with regard to movement of the examination subject. However Cartesian scanning is presently used nearly exclusively in clinical applications of MR imaging. The primary reason for this is that the realization of non-Cartesian k-space scanning schemes requires a more precise realization of the k-space trajectories then is the case for Cartesian scanning. This means that the degree of time synchronization of the various MR system components must be greater. In particular, it is known that a small time shift between the radio-frequency portion of an RF pulse and the envelope or the gradient fields is significant for the successful imaging by means of non-Cartesian scanning schemes of k-space.
An additional field in which a high degree of synchronization is necessary between the various MR system components is special RF pulses. By a special amplitude modulation of the associated gradient field for slice selection, such special pulses allow the spatial excitation profile to be designed so as to be particularly advantageous. It is thus possible to define the spatial excitation profile particularly sharply. Furthermore, by producing a uniform peak radio-frequency power it is possible to achieve a greater deflection angle of the magnetization from the steady state, and therefore to increase the signal-to-noise ratio given the same radio-frequency exposure. However, it is then necessary to temporally synchronize the gradient fields with the envelope and the radio-frequency portion of the RF pulses particularly precisely in comparison to conventional RF pulses.
For synchronization, methods can be used that enable relative time shifts of the individual components among one another to be measured directly in order to implement a calibration or time compensation based on these. For example, the rising edge of a gradient field is matched optimally precisely with the envelope of an RF pulse, or the envelope of the RF pulse is accordingly matched with the radio-frequency portion.
A synchronization of these different components conventionally takes place by means of a direct measured detection of the various component parameters, for example in the laboratory (what is known as error analysis). For example, in a design stage or development stage of the MR system the components can be affected such that the different timing signals are analyzed and the individual components are accordingly synchronized. It is thus possible to detect time shifts, for example with an oscilloscope or logic analyzer. Such methods have the disadvantage that different systematic errors can inherently occur due to deficiencies of this compensation measurement and of the compensation method. Such systematic errors are difficult to detect. It is thus not always possible to make all corresponding information of the tested components usable. Defined relevant temporal variables of a computer component for time control—for example a Field Programmable Gate Array (FPGA)—cannot be electronically tapped. Systematic errors can thus arise in the time synchronization: time shifts that have not been detected by the testing at the component level can occur in the implementation of an MR acquisition sequence. Furthermore, it is complicated to implement such methods for every MR system manufactured. A time shift that is MR system-specific and not specific to the model range can accordingly only be synchronized with difficulty by measures such as variation of cable lengths, for example.
Therefore, there is a need to provide an improved method for detection and compensation of a time shift of various components of a magnetic resonance system.